Mass spectrometers are well known in the art. To this juncture, mass spectrometers have utilized ionization methods in which the parent molecule lost or gained an electron, thereby resulting in a singly charged species.
There are a number of shortcomings associated with this prior art approach. First, electronic detection is difficult to achieve for those ions with a high mass-to-charge (m/z) ratio. Similarly, since most ions are singly charged, the mass range of the analyzer is limited.
Methods have been discovered which produce neutral parent molecules supporting multiple cations or anions. These new methods are disclosed in Dole, et al., Molecular Beams of Macroions, J. Phys. Chem., 1968, 49, 2240-2249. Particularly, electrospray (ES) technology has proven to be especially successful in creating multiple charging. This technique is disclosed in Yamashita, et al., Electrospray Ion Source. Another variation on the FreeJet Theme, J. Phys. Chem., 1984, 88, 4451-4459.
In accordance with these techniques, a mass spectrometry apparatus typically includes a number of elements: a liquid sample introduction device, a multiple charging apparatus, a mass spectrometer, and a data processing system.
The techniques associated with such an apparatus facilitates the formation of ions containing multiple adduct charges. As a result, ions have lower m/z values and thus are easier to detect and weigh than singly charged ions of the same mass, as done in the prior art. This technique extends the effective mass range of the analyzer by a factor equal to the number of charges per ion.
While this technique clearly has substantive advantages, it is difficult to interpret the resultant output. A plot of intensity versus m/z ratios results in a spectrum with multiple peaks.
Fenn, et al, Interpretinq Mass Spectra of Multiply Charged Ions, Anal. Chem. 1989, 61, 1702-1708 have done considerable work in interpreting such data. This paper is expressly incorporated by reference herein.
As explained in Fenn, resultant spectrums comprise a sequence of intensity peaks approximating a Gaussian distribution. Other general features include a width of approximately 500 on the m/z scale. This distribution is often centered at a value between 800 and 1200.
The individual peaks of an intensity versus m/z ratio spectrum represent the constituent ions. The number of charges on constituent ions for each peak differs from an adjacent peak by one elementary charge.
Fenn discloses an algorithm, referred to as "deconvolution" in the paper, which transforms the sequence of peaks for multiply charged ions to one peak located at the molecular mass M of the parent compound. Thus, the information possessed in the multiple peaks is greatly simplified into one peak corresponding to a molecular mass.
While an advance in the art, Fenn's approach has problems analyzing mixtures of components. This shortcoming arises because of the mutual interference of "side peaks" generated from different components in the transformed spectrum. A problem arises in determining whether such side peaks are a result of interference or represent a molecular mass. This problem is especially acute when one major compound dominates over the others, and thereby may conceal other molecular masses in the mixture being analyzed.